In the field of imaging technology, a significant need exists to increase image quality while at the same time maintaining low cost and low complexity. Unfortunately, these goals often conflict. For example, a particular class of imaging systems, known as sequential color systems, offer lower cost and complexity than other imaging systems, but at the sacrifice of some image quality.
Sequential color systems generate images by sequentially laying down red, green, and blue light in a single image frame, which typically lasts 1/60 of a second. In non-sequential color systems, the red, green, and blue light are laid down simultaneously. The nonsequential color systems, therefore, employ about three times the hardware and complexity of sequential color systems.
An excellent example of the distinction between sequential and non-sequential color systems is provided by spatial light modulator ("SLM") projection systems. One type of SLM imaging systems uses arrays of individual elements, such as deformable mirror devices ("DMDs"), to reflect light onto or away from a projection screen. In non-sequential color systems, three DMD arrays are used in parallel, one each for red, green, and blue light. In contrast, a sequential color system SLM device requires only one such array, with the red, green, and blue light sequentially reflected by the single DMD array. The need for three such arrays in the non-sequential color system triples the requirements for the DMD arrays and attendant hardware over the sequential color system.
As discussed above, however, sequential color systems have certain limitations. One such limitation is that of color separation. Color separation occurs in sequential color systems when an imaged object moves across a projection screen, and the human eye follows it. FIGS. 1-3 illustrate the problem of color separation.
FIG. 1a illustrates a projection screen 10 and an imaged object 12 that will move across the screen 10. In FIG. 1b, the various locations of object 12 are shown at five different time periods. Each of these time periods corresponds to one image frame. For a sequential color system that lays down color in the order of red, green, and then blue, the object 12 will be generated by first laying down the red, then the green, and then the blue. Therefore, as the object moves, the leading edge of the object 12 (with respect to its movement) will appear red, while its trailing edge will appear blue. This phenomenon is known as color separation.
FIGS. 2 and 3 illustrate how color separation occurs. As shown in FIG. 2, red is first laid down on the screen for about 1/3 of the imaging frame. After the red light is turned off, the green light is then turned on for about 1/3 of the color frame, and then the green is turned off and the blue is turned on for the remaining about 1/3 of the color frame. As shown in FIG. 2, the perceived intensity of the light dies away asymptotically after it is turned off. This asymptotic decrease illustrates the fact that the human eye has "memory" which allows it to continue to perceive light for a short period (a time constant) even after the light has disappeared.
The problem of color separation occurs in sequential color systems only when the human eye follows the moving object. As shown in FIG. 3a, if the human eye does not follow the moving object, then each image frame of light from the moving object will fall on different locations of the retina as the object moves. Thus, for each image frame, before red is perceived, the green and blue light will be laid down at one location, and the appropriate color will be perceived. Light from the next image frame will then fall on another location of the retina, and the appropriate color will again be perceived. However, as shown in FIG. 3b, if the eye follows the object, the red light from the object will always fall on one place on the retina, the blue light will always fall on another place on the retina, and the green light will always fall on still another place on the retina. Each of these places will be offset, due to the temporal separation of each color subframe. Therefore, the leading edge of the moving object will always appear red, while the trailing edge will always appear blue. This occurs because the eye moves before the blue and green are laid down at the leading edge, for example. The faster the object moves, the greater will this color separation be, since the distance the object moves from one image frame to the next will be greater.
The color separation becomes more and more complex the faster that an object moves. As described above, the leading edge will appear red and the trailing edge will appear blue, for sequential color systems that lay down red, then green, then blue. As the speed of the object increases, however, not only will the leading edge appear red, but the area of the object just behind the leading edge will appear to be a combination of red and green. Likewise, the area just ahead of the trailing edge will appear to be a combination of blue and green.
The problem of color separation is most notable when the moving object and its background are in high contrast. For example, a white object moving against a black background or a black object moving against a white background. Examples of situations where the human eye might follow such moving objects include sporting events where the human eye may follow a player whose uniform is in high contrast with the background, dance presentations where the human eye may follow the dancer, and other similar situations.